The present invention relates to a press device having an extended nip intended for pressing a running paper or paperboard web. More specifically, the invention relates to a shoe press of the type which comprises a support which supports a press shoe adjacent to a counter roll or another backing member in such a way that the press shoe and the backing member form an extended nip therebetween, and which also comprises a hydraulic or pneumatic arrangement for urging the press shoe toward the backing member in order to apply a pressure on the web passing through the nip.
The press device according to the invention particularly advantageously can be utilized for wet-pressing of moist paper or paperboard webs, but also for calendering or other finishing of dried paper or paperboard. The press device according to the invention can also be utilized for fiber webs other than paper and paperboard.
Shoe press devices, i.e. presses having an extended nip, have been employed for many years when manufacturing different paper and paperboard grades, primarily for wet-pressing in order to increase the dryness of the web, but also for calendering and other finishing in order to improve surface properties or other physical properties of the web.
As a result of their longer nips, shoe press devices provide a number of advantages in comparison to conventional roll presses, such as a higher dryness at the same nip pressure, or the ability to press at a lower at nip pressure while maintaining the same dryness, which is more gentle to the sheet.
U.S. Pat. No. 4,917,768 discloses a shoe press device in which the press shoe is supported via tubular sleeves rigidly affixed to and spaced apart on a support in a direction across the machine direction. The sleeves are received in cylindrical recesses in the press shoe in order to allow the press shoe to move toward or away from a counter roll such that the nip pressure can be varied. The disclosed shoe press comprises hydraulic jacks upstream and downstream of the sleeves for forcing the press shoe against the counter roll and for pivoting the shoe about an axis that extends in the cross-machine direction in order to vary the nip pressure in the machine direction. The sleeves fit somewhat loosely in the recesses in the shoe, and a resilient seal encircles each sleeve for sealing the interface between the sleeve and the recess. Accordingly, the press shoe of the shoe press disclosed in U.S. Pat. No. 4,917,768 is capable of pivoting relative to the support for varying the nip pressure in the machine direction.
However, it has been found that such shoe presses can be associated with certain problems. One such problem originates from the thermal expansion of the press shoe, which is a result of the heat generated by friction against the belt that runs over the press shoe and carries the paper or paperboard web through the press and by the hot hydraulic fluid which for different reasons is circulated through the shoe. The thermal expansion of the press shoe results in an elongation of the shoe in the cross-machine direction, which creates bending tensions in the support and hydraulic arrangement of the press shoe, which of course is undesirable.
In EP 0 933 471, corresponding to U.S. Pat. No. 6,083,352, the disclosure of which is incorporated herein by reference, a shoe press is disclosed which reduces the problems originating from the thermal expansion of the press shoe, since it has the ability to tolerate relatively large elongations of the press shoe across the machine direction, and also other deformations of the press shoe. The disclosed shoe press comprises a press shoe extending in the cross-machine direction along the entire width of a web running through the press, and a plurality of articulated hydraulic loading cylinders supported by a support and spaced apart along the shoe. The loading cylinders define working chambers which are pressurizable by hydraulic fluid, so as to enable the cylinders to urge the press shoe away from the support and toward a counter roll or other backing member for applying pressure to the web being carried through the nip defined between the shoe and the backing member. Each loading cylinder comprises a piston member disposed within a cylinder member. Either the piston or the cylinder comprises a two-part member having a first member fixed relative the press shoe and a second member fixed relative to the support, while the remaining piston or cylinder comprises a coupler.
In a preferred embodiment of the shoe press according to EP 0 933 471, the two-part member consists of first and second cylinders, while the coupler comprises a piston which is slidably received in the two cylinders. In an alternative preferred embodiment the two-part member instead comprises first and second pistons, while the coupler comprises a cylinder which surrounds both pistons.
The coupler of the shoe press disclosed in EP 0 933 471 sealingly engages at least one of the members, so that the first member is urged away from the second member in a loading direction when pressurizing the working chamber to cause the press shoe to be urged towards the backing member. In order to enable the loading cylinders to accommodate elongation of the press shoe across the machine direction, each coupler engages the respective first and second members at seals which enable the coupler to pivot relative to the first and second members about axes parallel to the machine direction. Accordingly, the press shoe is free to expand thermally in the cross-machine direction without causing bending of any piston and/or cylinder members of the loading cylinders.
Since the press shoe of the shoe press disclosed in EP 0 933 471 also can move or pivot in the machine direction relative to the support, the shoe press includes a stopping means which restricts the movement of the shoe forward in the machine direction.
In order to control the conditions in the nip of a shoe press, such as the pressure profile through the nip, it is usually desirable to be able to move the press shoe forward or backwards in the machine direction in order to be able to influence the pressure profile through the nip so that the nip pressure is highest in the beginning and lower in the end of the nip, or vice versa. When performing such a regulation of the pressure profile through the nip, the press shoe, in principle, will follow an arc-shaped path having its center at the central axis of the counter roll. This may result in a skewness in the machine direction between the parts included in the loading cylinders, i.e. between piston and cylinders or between pistons and cylinder. Such a skewness can generate forces both on the cylinder and on the shoe, which forces are directed in the machine direction or against the machine direction, depending on the direction of the skewness. If the skewness of the connecting member in a direction away from a possible stopping means (shoe support) is large enough, these forces may exceed the force directed towards the support, resulting in instability. If the connecting member is tilted in a direction toward the shoe support, forces which are directed toward the shoe support are generated, which increases the stresses on the shoe support.
Also in case a paper lump or the like unintentionally enters the nip, similar skewnesses can be generated and cause stresses on a shoe support.
Accordingly, the present invention provides a press device having an extended nip for pressing a running paper or paperboard web in which the press shoe unintentionally or intentionally can be tilted/pivoted around an imaginary axis across the machine direction with minimum skewness between the parts included in the loading cylinders, and without generating any excessively large forces directed towards a possible shoe support or the press shoe becoming unstable if the forces are directed against the machine direction (away from a possible shoe support).
In accordance with one preferred embodiment of the invention, a press device comprises a press shoe aligned across the machine direction and arranged for forming an extended nip in cooperation with a backing member for passage of the web therethrough, a support which movably supports the press shoe such that the shoe is movable toward and away from the backing member via a plurality of loading cylinders spaced apart along the press shoe for enabling application of pressure on the web during pressing. Each of the loading cylinders comprises a first cylinder member fixed on the press shoe, a second cylinder member fixed on the support, and a connecting member slidably engaged with and extending between the first and second cylinder members. The connecting member and the first cylinder member are slidable in relation to each other with a first length of stroke L1, and the connecting member and the second cylinder member are slidable in relation to each other with a second length of stroke L2. The connecting member has a length L3 in a direction corresponding to that along which the connecting member is slidable in relation to the first and second cylinder members. In accordance with the invention, the first length of stroke L1 is smaller than the second length of stroke L2.
Preferably, the first length of stroke L1 is less than half of the second length of stroke L2. Furthermore, it is preferred that the length L3 of the connecting member be larger than both the first length of stroke L1 and the second length of stroke L2. In one embodiment, the length L3 of the connecting member is larger than the sum of the first length of stroke L1 and the second length of stroke L2.
In accordance with a preferred embodiment of the invention, the connecting member sealingly engages the first and second cylinder members by contacting seals arranged between the connecting member and each of the cylinder members so as to form at least one working chamber within which pressure of a pressurizing medium can be increased in order to cause a lengthening of the loading cylinder or reduced in order to cause a shortening of the loading cylinder, and in such a way that a central axis C1 of the first cylinder member and a central axis C3 of the connecting member can be tilted to an angle relative to each other and relative to a central axis C2 of the second cylinder member as a result of a tilting of the press shoe during pressing. The lengths of stroke L1 and L2, the connecting member length L3, and geometrical positions of the contacting seals are such as to minimize an angle A2 between the central axis C3 of the connecting member and the central axis C2 of the second cylinder member during the tilting of the press shoe.
The press device in accordance with the invention preferably also includes at least one stopping member arranged on at least one of the first cylinder member and the connecting member so as to limit a maximum value of the first length of stroke L1.
Further objects of the present invention will become apparent from the following description, while the features enabling the further objects to be achieved are defined in the dependent claims.